A power pack [2] is located within a housing [3] on the platform [1] of a wind turbine generator tower. The power pack [2] is supplied from a fuel tank [4]. With the door [6] of the housing [3] open, the power pack [2] may be removed for servicing and repair by using a U-shaped support structure [8] to which an electric motor [11] is mounted. Two support rails [12] permanently located within the housing [3] support the housed power pack [2]. A chain [15] attached to the power pack [2] is driven by the motor [11] to remove the power pack [2] from the housing [3] by sliding the power pack [2] along the support structure [8]. A pulley arrangement reduces the necessary torque. After removing the power pack [2] from the housing [3], a crane [7] is used to lift the power pack [2] from the platform [1] and to lower it down to a sea vessel [17]. The crane [7] may also be used in the refilling of the fuel tank [4] by hauling a fuel line [18] from a sea vessel up to the platform [1] and connecting it to the fuel tank [4].